A rare Picasso painting at New York' City's Metropolitan Museum of Art was damaged with a 6-inch tear when an art lover lost her balance and stumbled into the $80 million work of art. link.
By Slate's Explainer.
The torn ends of the canvas can probably be lined up, and conservators can identify matching fibers on either side of the rip by inspecting them under a microscope. In general, you can expect the wefts in the fabric—that is, the crosswise yarns of the weave—to split at the site of the impact. The lengthwise warps tend to get stretched out, but they may not break.
The rip itself can be mended in a few different ways. First, the conservator can line up the torn ends and affix them to a new piece of fabric that lines the back of the painting. She might also try to attach the torn ends to each other using a method called Rissverklebung, in which individual fibers are rewoven back into place.
To reweave the warps and wefts, you have to figure out the proper placement of each individual fiber. Bits of paint that are stuck to the fibers must be glued in place or removed until the reweaving is complete. (Conservators map out the location of each paint flake they remove so it can be replaced in precisely the right spot.) Because an accident will stretch out some fibers and fray others, you sometimes have to tie off and shorten some threads while attaching new material to lengthen others. Threads attached to the back of the canvas will reinforce the seam.
Closing the tear is only the first part of the process. An accident can damage the painting in other places by stretching the fabric and distorting the image. To correct for these planar distortions, the conservators try to change the lengths of individual fibers or small patches of the canvas. Applied humidity can make a fiber expand across its diameter and shrink across its length—and tighten up distended parts of the weave.
Bits of paint that have fallen off the painting must also be replaced. Wynn might have surveyed the scene of the accident and saved any stray bits of paint for the conservators in a petri dish. Conservators have to touch up spots of missing paint with fresh material, color-matched to the surrounding area.
One more thing: Conservators always try to make their repairs reversible. That way, you won't cause any permanent damage to the work if you screw up, and someone can always try to improve on your work in the future.